(C) only speaks about whether it's a mark of success, not about whether it's a mark of "success
as an underground group".
That distinction may sound like it's splitting hairs, but that's kinda the whole gist of the argument, and the conclusion is specifically talking about "how many copies you sell isn't a good metric of whether you succeeded
as an underground group"
Success, as conventionally understood, would be selling a lot of copies of your recordings.
Success as an underground group is apparently different. The author is seemingly implying that you could sell a lot of copies, but not be successful as an underground group (because maybe you were too trendy).
But you also can't use how well recordings sell as an
inverse measure of success as an underground group. i.e., you can't assume that "the less well your recordings sold, the more success you had as an underground group". The author is implying that you could sell very few copies, but still be unsuccessful as an underground group (because you were incompetent).
In each of his examples,
1. sell a lot, but not successful as an underground group
2. sell a little, but not successful as an underground group
.... the author has left an idea unsaid
1. sell a lot -> too trendy --?--> not successful as an underground group
2. sell a little -> incompetent --?--> not successful as an underground group
It seems like success as an underground group involves being competent but not too trendy, whether or not you're crushing it with music sales.
(C) is making a bridge idea to conclusion, but it's grabbing something from the premises that is much more tangential.
The style of arguing is laying out a binary
This thing isn't a good metric for success at X.
When there is a lot of this thing, you may be unsuccessful at X.
When there's a little of this thing, you may be unsuccessful at X. (B) is using the keywords for "here's what happens in world X, here's happens in world not-X"
(C) is using a tangential thing that happens only in world not-X.
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This is what a logic robot would say to you in its dying breath.
It's the most beautiful sentence I've ever written. It made sense to me at the time, and now I'm convinced that it bends the mind into an incoherent pretzel.
Hope this helps.